Effects of the seasonal changes in a lake on the dynamics of a microcrustaceus: a six-year study in Lake Jacaré (MG)

One of the main objectives in ecologic studies is to identify patterns on the dynamics of populations and ecosystems. These patterns help us understand how different species can coexist, how they interact and which is the influence of environmental variables on the communities and populations. In order to identify these patterns, we need to assemble a set of environmental information obtained throughout a long period of time, in order to guarantee the certainty of our predictions. What makes this task difficult and sometimes almost impossible is maintaining the samplings in natural environments for a long time, due to the logistics, high cost and the maintenance of a technical staff.

An attempt to register these ecologic patterns in a more precise manner appeared with the implementation, in 1999, of the PELD program (Long Term Ecological Researches). With this project, the team of the Laboratory of Ecology of Zooplankton (Laboratório de Ecologia do Zooplâncton da UFMG) from UFMG, financed by the National Council of Scientific and Technological Development (Conselho Nacional de Desenvolvimento Científico e Tecnológico) and the Ministry of Science and Technology (Ministério da Ciência e Tecnologia) (CNPq/MTC) and by the Foundation for Research Support of Minas Gerais (Fundação de Amparo a Pesquisa do Estado de Minas Gerais (FAPEMIG) managed to monthly monitor Lake Jacaré, located in the surroundings of Rio Doce State Park (MG) during six years (2002 to 2007). The results obtained allowed the certification of a clear seasonal pattern on the abundance of Daphnia laevis, highly correlated to the seasonal changes on the water quality and to the presence of an invertebrate predator (the larvae of the insect Chaoborus sp.). The abundance of Daphnia laevis was always significantly higher during the winter, when the water temperature of the lake was practically the same from the surface to the bottom (thermally unstratified environment), and lower during the summer, when the water showed higher temperature on the surface and lower on the bottom (period of thermal stratification). This pattern was confirmed when the abundance of Daphnia laevis was correlated to the water nutrients (nitrogen and phosphorous), which were also always higher during the winter (period of thermal unstratification). The total circulation of the lake during this period, due to temperature difference, allows the distribution of the nutrients to the entire water column.

It was also observed that the abundance of Chaoborus sp. larvae, the main Daphnia laevis predator in the lake, increased with the availability of these organisms in the water column. These results reinforce the complex relationship between the quality of the environment and the interactions between the organisms, which can be deeply altered with changes in the general pattern of the ecosystems.

Another interesting finding was the registration of a high number of resistance eggs produced by the Daphnia. Whenever the environment's conditions adversely change, these organisms abandon the classic parthenogenesis reproduction (asexual reproduction) and start to produce, via sexual reproduction, an egg with very different characteristics, named resistance egg. These eggs can remain for years in the sediment, only hatching when the environment's conditions are once again favorable. The presence of these resistance eggs in the lake indicates that these microcrustacea are under constant stress, possibly due to the associated effect between predation and changes in the lake's water temperature. Considering the prediction of increase on the water temperature of the aquatic ecosystems caused by global climatic changes, the results of this study in Lake Jacaré are important for our comprehension of how ecosystemic changes can propagate intensively in natural environments, modifying dependency relationships established through years of coexistence.